Polytetrafluoroethylene, sometimes referred to as PTFE, is a widely available and widely useful fluoropolymer employed in a variety of applications such as a dry lubricant in the manufacture of, for example, medical hardware equipment, as a release agent, and as an industrial thickener. Polytetrafluoroethylene dispersed in a suitable solvent system may be applied onto a selected object or substrate by any number of known industrial coating processes. Dip coating, spray coating, and spin disk coating are among the application methods most useful to coat polytetrafluoroethylene. In accordance with these coating methods, a substrate or object to be coated with a fluoropolymer is exposed to the dispersed fluoropolymer, either by dipping the object or substrate directly into the dispersion, by spraying the dispersion onto the object or substrate, or by using centrifugal force to distribute the fluoropolymer about a spinning platform. The solvent or solvents used to create the dispersion are then removed, e.g. by ambient evaporation or by baking, leaving an even fluoropolymer coat. For a detailed description of these coating methods, see Stanley C. Zink, Coating Processes, 6 Kirk-Othmer Encyclopedia of Chemical Technology 386-426 (3d ed. 1979) (detailing methods of dip and spray coating) and U.S. Pat. No. 4,587,139 (describing spin disk coating).
Some dispersions useful for coating processes are commercially available. Generally, these dispersions contain between twenty and thirty weight percent solid polytetrafluoroethylene in a liquid phase solvent system containing one or more organic solvents or water. A commercially available dispersion of this type is DuPont Vydax.TM. Fluorotelomer Dispersion, a low molecular weight polytetrafluoroethylene dispersed in water or isopropanol. For use in coating applications, these dispersions are most often diluted with a suitable solvent to between one and three weight percent solids prior to processing.
Prior to the phase-out of chlorofluorocarbons (CFCs) mandated by the Montreal Protocol, many commercially available fluoropolymer dispersions were dispersed in CFC-113 (1,1,2-trichlorotrifluoroethane) as this fluid, apart from its environmental impact, is remarkably well adapted to process conditions; it is highly volatile, is extremely inert, is both nontoxic and nonflammable, and is highly compatible with a wide range of materials. CFC-113, for example, boils at 47.degree. C.
Currently available commercial dispersions, such as DuPont's Vydax.TM. Fluorotelomer Dispersion line, are sold in either isopropanol or water and are more poorly adapted to easy industrial use. Since both isopropanol and water are relatively nonvolatile (with boiling points of 82.degree. C. and 100.degree. C. respectively at standard pressure), fully diluted dispersions in these liquids can result in uneven coating due to their slow evaporative rates. Additionally, isopropanol-containing dispersions are flammable, and water containing dispersions can corrode ferrous metal substrates.
As a partial solution to the problems of incorporating water- and isopropanol-based PTFE dispersions into coating applications, some commercial manufacturers employ various hydrochlorofluorocarbons (HCFCs) alone and in combination with perfluorocarbons (PFCs) as preprocessing diluents for isopropanol- and water-based commercial dispersions. Although HCFCs possess the advantages of higher volatility and better solvency with a wider range of materials (HCFC 141 b, for example, has a boiling point of only 32.degree. C.), because of their high solvency, they often degrade polymeric substrates and because they contain chlorine, they contribute to stratospheric ozone depletion and are also scheduled for phase-out by the Montreal Protocol. PFCs are, like HCFCs, highly volatile, nontoxic, and nonflammable. But unlike HCFCs, PFCs are chemically inert with respect to polymeric materials, and since they are chlorine free, they do not contribute to stratospheric ozone depletion. PFCs therefore can be used as diluents in combination with HCFCs to reduce the aggressiveness of the HCFC action on polymeric substrates. But because this combination still employs HCFCs, their use only reduces, and does not eliminate, ozone depletion effects.